1. Field of the Invention
This invention relates to colour display apparatus comprising a colour display tube having a channel plate electron multiplier adjacent to a luminescent screen, the screen comprising a repeating pattern of three phosphor elements adapted to luninesce in different colours respectively, means for scanning a single electron beam across an input face of the electron multiplier in a television raster fashion to provide electron input thereto, colour selection electrodes disposed intermediate an output face of the electron multiplier and the luminescent screen which are operable to deflect the electron beam exiting from the channels of the electron multiplier and by appropriate control of which the electron beam can be directed selectively onto each of the plurality of phosphor elements, and circuit means including switching means connected to the colour selection electrodes for driving the display tube in accordance with received standard video signals.
2. Description of the Related Art
An example of such a colour display apparatus is described in published British Patent Application GB 2124017A (PHB 32887) corresponding to U.S. Pat. No. 4,560,898, issued Dec. 24, 1985. The colour selection electrodes used in this apparatus are in the form of a pair of electrodes for each channel of the electron multiplier arranged on opposite sides of the channel axis, or more precisely an aperture in an extractor electrode mounted on, and insulated from, the output face of the electron multiplier aligned with the channel which draws the electrons out of the channel. The pairs of electrodes are electrically insulated from one another and by suitably addressing the electrodes, the electron beam exiting from the aperture in the extractor electrode can be deflected to one side or the other so as to impinge upon respective ones of the plurality of phosphor elements of the repeating pattern to display selectively first and second colours, namely read and blue. In the undeflected state, that is with no potentials or the same potentials applied to the pairs of electrodes, the electron beam is directed onto the third phosphor element of the repeating pattern to produce a green display.
The channels of the electron multiplier, and likewise the aligned apertures of the extractor electrode, are arranged rectilinearly in horizontally spaced columns. The pairs of electrodes comprise strip electrodes with respective ones of the pairs of electrodes associated with adjacent channels being disposed next to one another between the columns and extending the height of the columns. Respective first and second electrodes of all the pairs of electrodes are interconnected electrically.
This display apparatus enables therefore a colour picture to be produced using a single electron beam which is scanned in raster fashion over the input face of the electron multiplier, the required line and field scan deflectors operating on the beam prior to reaching the electron multiplier.
The display apparatus may be used in order to display television pictures according to a conventional standard scanning format, for example the PAL standard of 625 lines, 2:1 interlace, 50 Hz field format where the input red, green and blue signals are derived from an RGB source such as a camera or telecine or from a PAL decoder. There is described in the aforementioned British Patent Application 2124017A and the corresponding U.S. Pat. No. 4,560,898, two modes of operating the display apparatus. In the first of these, the electron beam is scanned over the input face of the electron multiplier at the normal television line scan rate. The current multiplied beam leaves the extractor electrode at the same line scan rate and in the time whilst the beam is being emitted from the channel, the pair of electrodes associated therewith are switched in turn to the voltages necessary to achieve the necessary deflection to direct the beam onto the three, R, G and B, phosphor elements. If picture resolution and quality is not to be degraded this involves switching the voltage applied to the electrodes at a higher frequency than the picture element frequency (typically more than 2 times) whilst the intensity of the beam is switched from the luminance signal of one colour to another of the colours in synchronism with switching of the electrode voltages. Difficulties have been experienced in driving the apparatus in this manner in view of the effects of capacitance between the pairs of colour selection electrodes which causes problems in driving the electrodes with a required waveofrm, even in the case where a comparatively simple sinusoidal waveform is used, since it requires in order to obtain resonance an extremely small tuning inductance which is difficult to achieve in practice and involves a very high circulating current at a preferred cyclic selection frequency. Althrough cyclic selection at a lower frequency could be used to avoid to some extent these problems, this would give rise to a visible and resolution-limiting dot-sample structure in the picture which can also beat with the multiplier and screen structure to cause Moire effects.
In the second mode of operation described in the above-identified patent, a colour picture is obtained by producing successively red, green and blue colour fields in the time of one overall field period, for example 20 ms for a standard 25 frames/second television picture. To do this, the electron beam is scanned over the electron multiplier at three times the usual rate. The beam is modulated in turn for each set of three fields by the red, green and blue information. The voltages applied to the pairs of colour selection electrodes are switched in synchronism with the colour field to be displayed. A disadvantage with this mode of operation is that colour break up is possible with movement in the picture or of the viewer's eye. Moreover, the interlace could be disturbed by stray 50 Hz magnetic fields.